Apparatus and methods for cooling a cpu using a liquid bath

ABSTRACT

A system for cooling a CPU. The system has a tank for holding dielectric coolant in a liquid phase. The CPU is immersed in the coolant. A cover closes the tank. Electric pathways that convey data to/from the CPU traverse the cover. The electric pathways allow the CPU to exchange data with an external device. In this fashion, the CPU can perform data processing functions while being immersed in dielectric coolant.

FIELD OF THE INVENTION

The invention relates to a system for cooling an electronic dataprocessor such as the CPU by immersion in a liquid bath. Specifically,the invention relates to the structure of the cooling system and tomethodologies for controlling the cooling system to adequately cool theCPU.

BACKGROUND OF THE INVENTION

Electronic data processing devices such as servers that are commonlyavailable today cool the central processing unit (CPU) by using forcedair circulation. This method of cooling is satisfactory when the powerconsumption of the CPU is comparatively low. However, it becomes alimiting factor when the power consumption of the CPU significantlyincreases. Since the data-processing capability of the CPU is directlytied to its electric power consumption, the ability to adequately coolthe CPU is an important design requirement.

With the expectation that the data processing capacity of CPUs willsignificantly increase over time, it is plain that novel coolingtechniques are required in order to enable the devices to operate withintheir thermal limits.

SUMMARY OF THE INVENTION

In a first broad aspect, the invention relates to a system for cooling aCPU, comprising a tank for holding dielectric coolant in a liquid phaseand for receiving the CPU that is immersed in the coolant, a cover forclosing the tank and an electric pathway through the cover forcommunicating data signals to the CPU.

In a second broad aspect the invention provides a method for cooling aCPU, the method including immersing the CPU in a bath of liquid coolantand varying an immersion depth of the CPU in the liquid coolant withvarying cooling requirements of the CPU.

In a third broad aspect, the invention provides a method for cooling aCPU, the method including immersing the CPU in a bath of liquid coolantcontained in a tank and varying a vapor pressure of coolant in the tankwith varying cooling requirements of the CPU.

In a fourth broad aspect, the invention provides a method for cooling aCPU, the method including immersing the CPU in a bath of liquid coolantcontained in an enclosed vessel having a wall and passing data viaelectric signals through the wall to the CPU for processing.

In a fifth broad aspect, the invention provides a method for cooling aCPU having a heat transfer surface, the method including immersing theCPU in a bath of liquid coolant and perturbing coolant bubbles at theheat transfer surface to induce the bubbles to detach from the surface.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of the electronic module cooling system according toan example implementation of the invention;

FIG. 2 is an enlarged view of the main cooling chamber of the systemshown in FIG. 1 also illustrating a buffer tank for the primary coolingliquid;

FIG. 3 is an enlarged view of a cooling liquid make-up tank designed forreplenishing the main cooling chamber with primary cooling liquid;

FIG. 4 is an enlarged view of the lid of the cooling chamber and theassociated fluid, data and electrical connections;

FIG. 5 is yet another enlarged view of a component shown in FIG. 1,which is a piston used to regulate the level of coolant in the maincooling chamber;

FIG. 6 is an enlarged view of the primary heat exchanger and theassociated make-up tank of the cooling system in FIG. 1;

FIG. 7 is a block diagram illustrating a computing platform forexecuting software that implements a control system for the coolingsystem;

FIG. 8 is a flowchart of a process for computing what the temperature ofthe CPU will be on the basis of the data processing load of the CPU;

FIG. 9 is a flowchart of a process for computing what the temperature ofthe CPU will be on the basis of the electrical power consumption of theCPU;

FIG. 10 is a flowchart illustrating the process for selecting betweendifferent operational modes of the cooling system;

FIG. 11 is a flowchart illustrating a process for controlling the CPUoperation on the basis of the cooling capacity of the cooling system;

FIG. 12 is a flowchart illustrating a process for determining theconductivity of the primary cooling liquid;

FIG. 13 is a flowchart illustrating a process for determining thecooling capacity of the primary cooling liquid;

FIG. 14 is a flowchart illustrating a process for regulating the boilingpoint of the primary coolant.

EXAMPLE OF IMPLEMENTATION OF THE INVENTION

An example implementation of the invention is illustrated in the annexeddrawings. With specific reference to FIG. 1, the invention provides acooling system for an electronic component that uses an electronicdevice, including a CPU. The cooling is performed by immersion of theCPU in a liquid coolant. The liquid coolant is dielectric to avoidshort-circuiting the electrical connections between the CPU and theassociated electronic components. Examples of suitable dielectriccooling liquids will be provided later.

The cooling system generally operates according to different heatabsorption modes. For example, one mode uses conduction to absorb heatthat is generated by the CPU. Thus, thermal energy is directed from theCPU to the adjoining liquid, which has the effect of elevating thetemperature of the cooling liquid. To prevent the cooling liquid fromoverheating, it is circulated through a cooling loop.

Another heat absorption mode uses phase change to take up thermal energyfrom the electronic device. When the temperature of the electronicdevice exceeds the boiling point of the cooling liquid, it will causethe liquid to evaporate. As the vapor is cooled, it condensates andreturns to the main cooling chamber for another evaporation cycle.

The system can be operated in either of the aforementioned modes or in acombination thereof depending on the cooling requirements.

The cooling system is generally designated by the reference 10 inFIG. 1. The cooling system 10 includes a main cooling chamber 12 inwhich the CPU and the associated electronic circuitry are immersed incooling liquid. The cooling system 10 further includes a liftingmechanism 14 for raising the lid of the cooling chamber and alsoelectronic module being cooled. Finally, the cooling system 10 includesa buffer tank section 16 for the cooling liquid.

The structure of the main cooling chamber 12 is best shown in FIG. 2.The main cooling chamber 12 is essentially a vessel designed to hold apredetermined quantity of cooling liquid and it is dimensioned in orderto receive the electronic module 18 to be cooled. The electronic module18 includes the CPU, which typically would be the main heat generationcomponent of the module. A data server is an example of such electronicmodule 18.

The vessel can be made of any suitable material but it is preferred touse a metallic component in order to isolate the electronic module 18from external electromagnetic interferences. In a possible variant, thevessel can be made of a composite material and a suitableelectromagnetic shielding, such as copper meshing can be applied on it.The material of the vessel itself or the shielding material iselectrically connected to ground during installation.

The vessel includes an outer thermal insulation layer 22 to prevent orat least reduce thermal exchange between the cooling system 10 and theexternal environment. This insulation layer 22 prevents a cooling loopin the main cooling chamber 12 to absorb external heat, which will makeit less effective in cooling the cooling liquid in the main coolingchamber 12. The thermal insulation layer 22 may include a layer ofpolymeric foam or any other suitable material having the desiredinsulation properties.

The main cooling chamber 12 includes an internal frame 23, outside ofwhich is provided a cooling jacket 24. The frame 23 is used to supportelements surrounding the main cooling chamber 12 (such as levelcontrols, ultrasonic equipment, conduits, etc.). The jacket 24 is anarea in which secondary coolant can flow to cool the primary coolant inthe main cooling chamber 12 and that is in direct contact with the CPU.The jacket 24 can be designed as a void volume that is filled withsecondary coolant. The flow of secondary coolant in that space allowsremoving heat from the primary coolant via thermal transfer through theinternal frame 23. For that reason, the material of the internal frame23 should be selected to facilitate heat transfer. Metallic material ispreferred.

Wrapping the main cooling chamber 12 with a conduit in which secondarycoolant flows can also form the jacket 24. For instance, the conduit,which can be made from copper or other metallic material, is coiledaround the frame 23. The conduit can be brazed to the frame 23 toenhance heat transfer.

The jacket 24 has a secondary coolant inlet and a secondary coolantoutlet. The inlet and the outlet can be placed in different positionsbut is generally preferred to locate them such that they are oppositelocations on the main cooling chamber 12 to extend the fluid pathbetween them. In this fashion, the heat take-up ability of thearrangement is enhanced due to increased distance and also due to thecoil-like path that the liquid develops as it passes in the jacket.

The main cooling chamber 12 has a lid 26. The lid 26 closes the maincooling chamber 12 to prevent primary coolant from escaping especiallywhen the coolant is in a gaseous phase. The lid 26 also constitutes asupport structure for the electronic module 18 and further constitutes acondenser for the primary coolant. Also the lid 26 constitutes aninterface for electrical, data and secondary coolant connections.

The structure of the lid 26 is best shown in FIG. 4. It has a generalconfiguration that matches the top portion of the main cooling chamber12 such that it can partially fit inside. The lid 26 physically rests onthe upper edge of the vessel defining the main cooling chamber 12. Asuitable gasket 28 is provided to create a fluid tight seal. A gasketmade of neoprene is preferred.

The lid 26 has a bottom surface 30 that constitutes a condenser for thegaseous primary coolant that is evaporating and rising up. The bottomsurface 30 is thus chilled to condense the primary coolant vapors and itis also configured to induce the condensed droplets to migrate over thechilled surface such as to cool them as much as possible. Specifically,the bottom surface 30 has an incline rather than being perfectlyhorizontal. In the example shown the incline is toward the outer edge ofthe condenser and fall into the body of primary coolant close to thewall of the vessel which is cooled by the jacket. As primary coolantvapors condense, they will flow under the effect of gravity toward theouter edge. As they flow, the liquid material will spread over thecondenser forming streaks which is likely to increase the heat transferand further chill the condensate.

Note that in a possible variant, the incline may be orienteddifferently. Instead of being oriented toward the periphery, the bottomsurface 30 may slope toward the center. This variant is expected tofunction about the same way. Other arrangements are also possiblewithout departing from the spirit of the invention.

The lid 26 has an internal jacket for secondary coolant circulation. Thejacket is a fluid path 32 having an inlet 34 and an outlet 36. The inlet34 and outlet 36 can be located at opposite ends of the lid 26 such thatthe fluid undergoes helicoidal movement within the lid 26, therebyproviding even cooling throughout. The configuration of the jacket issuch that it is close to the bottom surface 30 to provide as muchcooling as possible.

A secondary coolant feed line 38 connects to the inlet 34 to feedpressurized cooling media, such as water. The warmed up secondarycoolant is removed via an outlet 40.

A lift hanger 42 is provided in the central portion of the lid 26 toallow raising and lowering the lid 26. The lift hanger 42 alsoconstitutes a feed through for the passage of secondary coolant,electrical supply and data communication with the electronic module 18.The lift hanger 42 is a generally elongated cylindrical structure thatis centrally placed on the lid 26. It has at its top end 43 a projectionfor connecting to a lifting mechanism that will be described in detaillater. Internally, the lift hanger 42 defines an inner peripheralchamber 44 for receiving the electrical and the data connections and acentral chamber 46 for receiving the secondary coolant conduit.

The electrical supply cable 48 supplies electrical power to theelectronic module 18. The electrical supply cable 48 has a series ofpass-through fittings 50, 52 that establish a fluid tight connectionpreventing the escape of primary coolant vapors through the lift hanger42. The data connection is configured in a similar fashion with fittings51, 53. The data connection cable 54 allows the electronic module 18 toexchange data with external components. To reduce the likelihood of databeing corrupted as a result of electromagnetic interference, the dataconnection cable 54 is provided with a suitable shielding.

The lower end of the lift hanger 42 has a yoke 56 from which hangs theelectronic module 18. The mechanical connection between the yoke 56 andthe electronic module 18 may be made by a steel cable or by any othersuitable arrangement, such as a rigid plate.

Referring back to FIG. 1, the cooling system 10 is has a lid liftingmechanism 14 designed to automatically raise the lid 26 to open the maincooling chamber 12. Raising the lid 26 would typically be required whenmaintenance needs to be performed on the electronic module 18 or on themain cooling chamber 12. When the lid 26 is raised, the electronicmodule 18 that is suspended from it is removed from the primary coolantbath.

As it will be discussed later, the procedure for lifting the electronicmodule 18 from the coolant bath can be performed automatically. Duringthe process, the coolant level adjustment mechanism is de-activated.Without such de-activation, the automatic control system will observe aliquid level drop resulting form the removal of the electronic moduleand will attempt to compensate by commanding valves to flow additionalprimary coolant in the main cooling chamber. If the level is thusraised, the insertion of the electronic module back in the body ofliquid will cause an overflow.

The lid lifting mechanism 14 includes a jackscrew 58 connected to motor60. When the motor 60 is powered and turns, the jackscrew 58 moves up ordown, depending on the direction of rotation. Therefore, the jackscrew58 acts as a linear actuator. The jackscrew 58 is connected via a rotarycoupling to the top end 43 of the lift hanger. As the jackscrew 58 movesup, the lid 26 is raised to open the main cooling chamber 12.

A guiding structure 62 is provided to guide the vertical movement of thelid 26. The guiding structure 62 includes a pair of horizontally spacedapart vertical rails 64. Spring biased rollers 66, best shown in FIG. 4,mounted on the lid 26 engage the rails 64 and ride on the rails 64 asthe lid 26 moves up and down. The rails 64 thus guide the lid movementand prevent it from swinging or otherwise moving horizontally.

Above the lid 26 is provided a primary coolant make-up tank 70, which isillustrated in greater detail at FIG. 3. The primary coolant make-uptank 70 is a source of primary coolant to replenish the primary coolantlevel in the main cooling chamber 12. Since the main cooling chamber 12is closed, which prevents loss of primary coolant vapors, the level ofprimary coolant will not vary much during the operation of theelectronic module 18. However, some loss of primary coolant may occurover time and the make-up tank 70 is designed to compensate for thatloss.

The make-up tank 70 can also be used to provide an emergency coolingsource in the event the level of primary coolant in the main coolingchamber 12 suddenly drops, which may occur as a result of rupture ofcoolant conduit. If the coolant loss is rapid the CPU in the electronicmodule 18 may overheat. The make-up tank 70 thus provides an emergencysource of coolant that can be directed to the main cooling chamber toprovide sufficient cooling medium until the electronic module 18 hasbeen shut down.

The coolant make-up tank 70 is located immediately below the electricmotor 60 and it is spaced apart from the lid 26 by a distance sufficientto provide for a sufficient range of movement for the lid 26. Bylocating the make-up tank 70 above the main cooling chamber 12, coolantcan flow by gravity, avoiding the use of a pump. Note that this is notessential to the invention and the make-up tank 70 can be placed in anyother suitable location.

The make-up tank 70 has a primary coolant supply line 72 to keep themake-up tank 70 full. A series of valves control the flow of coolant inthe line 72 to the make-up tank 70. Specifically, there are manual ballvalves 76 and an electric valve 78 which can automatically trigger thereplenishment of the make-up tank 70 through the line 72.

The level of primary coolant in the make-up tank 70 is determined by alevel sensor 80, which can include floats or any other suitable levelsensing device to sense a low coolant level and a high coolant level.

Primary coolant is directed from the make-up tank 70 to the main coolingchamber 12 via a conduit 82. A strainer 84 in the make-up tank 70filters the coolant to remove impurities. The conduit 82 can deliver thecoolant to the main cooling chamber 12 via the lid 26, passing throughthe lift hanger 42 or through any other suitable location. The conduit82 may have a portion that is flexible to accommodate the movement ofthe lid 26. Valves, including a manually operated ball valve 84 and anelectric valve 86, control the flow of the coolant in the conduit 82.

FIG. 5 illustrates an arrangement for controlling the level of coolantin the main cooling chamber 12. The level of coolant changes the bubbleformation dynamics when the coolant transitions from the liquid phase tothe gaseous phase and thus can be used to control how heat is beingextracted from the CPU. The example of implementation uses a mechanicalarrangement based on a piston that displaces a column of coolant,however other arrangements are possible without departing from thespirit of the invention.

In the example shown, a cylinder 90 is provided beside the main coolingchamber. The cylinder 90 is hollow and it is vertically oriented. Thecylinder 90 connects to a piston venting 91 through which the maincooling chamber 12 can be filled with coolant. A jackscrew 92 is placedinto the cylinder 90. The jackscrew 92 is driven by an electric motorand gear arrangement 94. A piston 96 is threadedly engaged on thejackscrew 92 such that as the jackscrew 92 rotates, the piston 96 iscaused to move up or down, depending on the direction of rotation of thejackscrew 92. In the arrangement shown, the main cooling chamber 12 canbe filled by positioning the piston 96 before the piston venting 91.When filling of the main cooling chamber 12 is complete, the piston 96is moved below the piston venting 91 and can be operated to control thelevel of coolant in the main cooling chamber 12.

The cylinder 90 is in fluid communication with the main cooling chamber12. As best shown in FIG. 1, the lower end of the cylinder 90 connectsvia a short conduit with the bottom end of the main cooling chamber 12.The arrangement is such that the level of coolant in the main coolingchamber 12 and in the cylinder 90 is at the same level, during thenormal operation of the cooling device. If the piston 96 is drivendownwardly it will displace coolant to the main cooling chamber 12 andforce the coolant level in the main cooling chamber 12 to rise.

The further the piston 96 is moved down, the more the level in the maincooling chamber 12 rises.

When the movement of the piston 96 is reversed, the level in the maincooling chamber 12 drops until the level of coolant in the cylinder 90and the level in the main cooling chamber 12 equalize.

Note that the coolant level control, in addition to regulating thecoolant pressure head on the CPU can also be used as a mechanism toregulate the vapor pressure in the main cooling chamber 12. If the maincooling chamber 12 is gas tight and no vapors are allowed to escape, avariation of the coolant level will change the pressure in the maincooling chamber 12. When the level of coolant in the main coolingchamber 12 is raised, the vapor pressure will increase. In turn, thiswill affect the temperature at which the coolant will boil; the higherthe pressure the higher the boiling point.

Also, the piston 96 can be used as an emergency coolant supply to themain cooling chamber 12 should the coolant level in that chamberunexpectedly drop. In such case, the piston is lowered to expel liquidfrom the cylinder 90 to the main cooling chamber 12. The piston 96 canbe actuated in conjunction with the make-up tank 70 to rapidlycompensate for a loss of coolant.

As further shown in FIG. 1, the main cooling chamber is provided with aseries of bubble flow management devices designed to accelerate thebubble separation from the interface cooling liquid/CPU such as to avoidthe thermal insulation effect that the gas pocket in the bubble has ifit remains attached to the interface.

The bubble flow management devices includes a baffle designed to createa Venturi effect and accelerate the gaseous flow upwards, hence awayfrom the interface to be cooled. The baffle 100 is essentially a hollowstructure in which the electronic module 18 is located. The structure ischimney shaped, and has progressively narrowing cross-sectionalconfiguration in the vertical direction. As the bubble stream isreleased from the CPU interface, it increases its velocity as flowcross-section diminishes due to the baffle. This has the effect ofincreasing the velocity of the stream upward and creates suction, thuspromoting the flow of fresh liquid coolant toward the CPU interface.

Yet another possible bubble flow management device, which can be used inconjunction with the baffle 100 or independently of the baffle 100, isto provide mechanical agitation to promote bubble separation from theCPU interface. The mechanical agitation disturbs the bubbles and it isbelieved that it weakens the bubble attachment with the CPU interface.Mechanical agitation can be provided in different ways. One is to impartvibration to the CPU itself, which can be achieved, by imparting anoscillatory movement of small amplitude to the CPU. The frequency of themovement does not need to be high, in the order of 5 Hz or slightly moreto obtain a benefit. Objectively this is the least desirable optionbecause of possible CPU durability concerns resulting from theoscillatory movement.

Another way to create mechanical agitation is to impart waves into thecoolant. An ultrasonic generator 102 shown in FIG. 2 can accomplishthis. The ultrasonic waves travelling in the body of liquid coolantinteract with the bubbles adhering to the CPU interface and promoteseparation of the bubbles. The location of the ultrasonic generator inthe main cooling chamber 12, the frequency of operation and themagnitude of the oscillations are design considerations within the reachof a person skilled in the art. Note that the high frequency waves inthe coolant enhance the heat transfer from the CPU to the coolant inaddition to reducing the bubbles adherence.

The ultrasonic generator can be designed to generate radiation having avariable intensity. The range of radiation energy can vary up to 3400Wm².

Yet another way to provide mechanical agitation is to direct coolantjets at the CPU interface in order to create coolant flow to entrain thebubbles. Coolant jets can be placed in any suitable location and fedwith pressurized coolant by a pump. The orientation of the jets shouldbe such as to create a flow of coolant generally in the plane of the CPUinterface to continuously ‘wipe’ the bubbles. Advantageously, thecoolant so injected can be delivered directly from a heat exchanger,such that it is chilled and can further take heat away from the CPUinterface.

The main cooling chamber 12 also includes a series of coolant flowmanagement devices to assist with proper distribution of chilled coolantsuch as to enhance heat take-up from the CPU. The coolant flowmanagement devices include a flow sparger 104 located immediately infront of the chilled coolant inlet 106. The flow sparger is a physicalstructure that distributes the flow of chilled coolant within the maincooling chamber more uniformly. The coolant flow management devicesfurther includes a perforated plate that is located downstream the flowsparger 104, relative to the direction of chilled coolant from the inlet106. The effect of the perforated plate is to reduce flow turbulencesuch that the flow becomes laminar.

The primary coolant path includes a buffer tank section 16 illustratedin greater detail on FIG. 6. The buffer tank section 16 includes a tank200 that receives hot coolant from the main cooling chamber 12 at theinlet 202. The tank 200 defines an enclosure that has the desiredvolume, which is provided with an outer cooling jacket 204. The jacket204 is a pathway for secondary coolant to chill the primary coolant inthe tank 200. It is preferred that the coolant in the buffer tanksection 16 remain in a liquid state. As such, the cooling system 10 canrely on the buffer tank section 16 to provide sufficiently cool liquidwhen needed. The secondary coolant may be water or any other suitablemedium.

As discussed earlier in connection with the main cooling chamber 12, thejacket 204 of cooling tank 200 may have different configurations. Onepossibility is to provide on the tank 200 a spiraled conduit in whichsecondary coolant flows and which is in thermal exchange relationshipwith the primary coolant. Yet another possibility is to create acontinuous hollow space that completely encloses the tank 200 and whichhas the advantage of allowing for a greater secondary coolant volume toegress the jacket.

Secondary coolant flows through the jacket 204 by entering via an inlet206 near the top of the tank 200 and leaving by an outlet 208 near thebottom of the tank 200. Chilled primary coolant is removed from the tank200 by a pick up tube 210 that reaches near the bottom of the tank 200,where the chilled coolant accumulates in light of its higher density bycomparison to the hot coolant. The chilled coolant is pumped back to themain cooling chamber 12 via a conduit and a pump arrangement leading tothe inlet 106.

The buffer tank section 16 has a local make-up tank 300 to add primarycoolant and compensate for loss due to evaporation. The local make-uptank 300, in addition to supplying coolant can also be designed to coolit. A secondary coolant jacket 302 through which flows secondary coolantsuch as water provides the cooling assist function.

A conduit 304 establishes a fluid communication between the localmake-up tank 300 and the tank 200. By locating the secondary make-uptank 300 at a higher elevation than the tank 200, primary coolant willflow under the effect of gravity directly into the tank 200.

A possible option is to pressurize the local make-up tank 300 such thata high volume of chilled coolant can be injected into the tank 200 whenthe circumstances require it. In this example, the local make-up tank issealed at the top and a pressurized air pocket is created on top of theliquid body. When the conduit 304 is opened, the added pressure assistsexpelling the primary coolant from the local make-up tank 300 to thetank 200.

The preferred option, however, is provide the conduit 304 with a pumpfor controlling of the flow of coolant.

To facilitate the heat transfer from the CPU surface to the liquidcoolant it is possible to provide the CPU surface with a treatment orconfiguration facilitating bubble formation and bubble release. Anexample of such surface configuration is to create a porous layer thatwill increase the surface area of the CPU/coolant interface. Theporosity and the thickness of the porous layer may vary. For instance,the pores are open pores to allow bubbles to escape. In addition, it isgenerally preferred to dimension the pores such that the average poresize is larger than the average bubble size. In this fashion, bubblesare less likely to become trapped in the porous network. Bubbleformation may induce an isolation layer due to the fact that the heattransfer from the surface of the electronic module 18 is less throughgas than through liquid. The bubble starts small and increases in sizeuntil the point where the force of differential density is larger thanthe force of adhesion of the bubble surface to the CPU surface. Hencethe bubble should be carried away as fast as possible once created.Another feature of the porous layer is to increase the heat transfercoefficient, thereby increasing the heat flux at the CPU/coolantinterface.

The porous network can have a random and generally uniform poredistribution or the pore distribution can be controlled to create apore-size gradient. The pore size gradient is such that the pore sizeincreases with the distance from the CPU surface. In other words, thepores that are closer to the CPU surface are the smallest and movingfurther away from the CPU the pores become increasingly larger. Smallpores create a larger heat exchange surface and also provide morenucleation sites for bubble formation. As bubbles are created andreleased from the smaller pores, they travel through larger pores whichowing to their size provide a larger escape pathway to prevent bubbletrapping. The porosity gradient employed should allow for high heattransfer and ease of bubble extraction at the CPU/coolant interface.

The material used to make the porous network is thermally conductive.Metallic material is a material of choice. A porous network can bemanufactured from copper by using the process described in the U.S. Pat.No. 6,660,224 entitled “Method for making open cell material” andassigned to the National Research Council of Canada.

The porous layer can be adhered to the surface of the CPU by usingvarious techniques, such as by clamps that press the porous networkagainst the CPU surface to increase the heat transfer potential, orusing bonding components that, in addition to providing a mechanicalattachment, create a pathway for heat to travel toward the porousnetwork. Brazing is an example of such bonding component, which can beused to the extent the surface of the CPU, has a metallic coating on it.

An example of a surface treatment to facilitate bubble release, whichcan be applied to the CPU surface, the porous network surface or both,is a treatment to reduce the surface tension. Without intent of beingbound by any particular theory, it is believed that a lower surfacetension at the interface at which bubbles nucleate, facilitates bubblerelease. An example of a surface treatment to reduce the surface tensionis to apply a surfactant. Again, the surfactant can be applied to theCPU surface, the porous network surface or both. The choice of thesurfactant should take into consideration its compatibility with theprimary coolant.

Since the primary coolant is in direct contact with the CPU andassociated electrical components and connections, the coolant should bedielectric to prevent shorts circuits. The chemical sold by 3M under thetrademark Novec is an example of liquid that has the necessarydielectric properties to be used in applications in which the coolant isin direct contact with the electronic circuitry.

The primary coolant liquid can be engineered with a specific boilingpoint at a temperature selected according to the cooling requirements.Since the phase transition from liquid to vapor takes-up a significantamount of energy, the boiling point is selected to be lower than themaximal operational temperature of the CPU. In other words, if thetemperature of the CPU progressively increases, the coolant should startboiling before the point at which a critical temperature is reached andthe CPU must be shut down or throttled down to prevent it fromoverheating. The temperature delta, which is the difference between theCPU critical temperature, which is considered to be the upper limit ofits operational temperature range and the boiling point, should beaccording to the OEM specifications. It is however preferred that theboiling point of the primary coolant liquid be below the CPU criticaltemperature.

A possible refinement is to formulate the primary coolant in such a waythat it provides phase transition from liquid to vapor at differenttemperatures. In a specific example, this can be achieved by mixingliquids having different boiling points. The family of Novec productsreferred to earlier can be engineered to provide a range of boilingpoints so it is a matter of selecting the proper liquid composition toprovide the desired phase transition temperatures.

Coolant with multiple boiling points is preferred because it provides amore gradual thermal energy absorption than a liquid having a singleboiling point. A single boiling point invokes a significant heat take-upmechanism and it is not a gradual process. It is rather a step process.With multiple boiling points the mechanism is more progressive. Albeitit still has a step-like nature, there are multiple steps so it ispossible to operate between steps.

For example, the liquid coolant can be a mixture of two liquids of theNovec family having boiling points A and B respectively, where A islower than B. As the temperature of the CPU increases, the liquid withboiling point A will undergo phase change and will provide an enhancedcooling action. The additional cooling may thus suffice to stabilize theCPU temperature. Should increased cooling be further required, thefraction of the coolant with boiling point B will start changing phase.At that point, both coolant fractions will be boiling.

In a specific example, the boiling points can be selected such as tostraddle the operational temperature of the CPU. In other words, duringsteady state operation, the CPU is at a temperature that exceeds theboiling point A (which is assumed to the lowest) and that coolantfraction is boiling. The fraction having boiling point B (which is thehighest) starts to change phase when a higher temperature is reached. Aswith the previous example, the boiling point B is at or slightly belowthe critical temperature such as to provide additional cooling beforethe temperature reaches a point where the CPU has to be shut down.

Another advantage of using coolant engineered with multiple boilingpoints is the capability of the fraction of the coolant that is stillliquid to condensate at least in part the gaseous fraction. Since thedifference of temperature between the boiling points can be significant,in the order of 10 degrees Celsius or more, the bubbles of theevaporating fraction have to travel through the liquid medium to reachthe surface of the coolant body. That liquid medium has the ability totake up more heat, as its boiling point is higher. The cooling effectprovided by the coolant that is still liquid on the vapor component may,in certain circumstances, suffice to completely condensate the vapor.Thus, little or no bubbles will break the surface.

The fractions having different boiling points may have the same density,in which case they will likely mix uniformly or different densities.

Different density cooling fractions can also be used when they havesimilar boiling points. In this situation, the body of coolant in themain cooling chamber 12 is stratified and there is a lower densityfraction on top with a higher density fraction below. Assuming that thehigher density fraction starts to boil first, the vapor will travelthrough the lighter density fraction and assuming this fraction issufficiently cool, it will condensate at least in part the vapors.

FIG. 7 is a block diagram illustrating the principal components of acontrol system to control the operation of the cooling system 10.Generally speaking, the control system includes a CPU 700 incommunication with a machine-readable storage 702, commonly referred toas memory. The memory 702 stores program code for execution by the CPU700. The program code defines the logic for controlling the operation ofthe cooling system 10. Also, the memory 702 stores data that isprocessed by the program code, such as data derived from the varioussensors 704 of the cooling system 10. Examples of sensors include liquidlevel sensors, temperature sensors, flow sensors, etc. Thus, datagenerated from the sensors is stored in the memory 702 for subsequentuse. For example, temperature sensors may communicate temperaturereadings from the electronic module 18 (or, alternatively, the liquidcoolant) in order to derive heat generation information as will bediscussed below. The software executed by the CPU processes the datastored in memory 702 in order to output control signals to control thecooling system 10. The control signals regulate the operation of controlcomponents 708 (i.e.: pumps, valves, piston level regulator, tank coverlifting gear, etc) as it will be discussed in greater detail below.

The control system also has an input/output interface 706 through whichdata generated by the various sensors 704 is communicated to the memory702. In addition, the control signals generated by the CPU 700 aredirected to the various control components 708 via the input/outputinterface 706.

Note that the CPU 700 is different from the CPU that is being cooled.This is a preferred mode of operation. In theory, the CPU that is cooledcan also execute the code for controlling the cooling system 10 and thusprovide the control functions, however this approach may not always workwell since the CPU is essentially controlling itself. For instance, ifan overheating condition arises and the CPU shuts itself down, thecontrol function of the cooling system 10 also shuts down, which isundesirable.

To provide adequate cooling the control system needs to assess what thecooling requirements are. This can be done in two ways. The first way isa reactive way and the second is a pro-active way. The reactive wayrelies on measurement of certain parameters that indicate what thetemperature of the CPU is and, in response to those parameters, theoperation of the cooling system 10 is adjusted accordingly. Thisapproach works well when the temperature of the CPU varies relativelyslowly so there is sufficient reaction time. Also note that inherently,the cooling system 10 can rapidly absorb a certain amount of heat tocompensate for a rapid heat rise of the CPU, without any action beingtaken. The liquid coolant in the main chamber is a large thermal bufferand in event of a rapid heat rise of the CPU, the coolant willimmediately provide cooling by boiling or simply heat conduction to keepthe temperature at a manageable level. This provides enough time for acontrol action to be implemented. A control action can be, for example,switching between different cooling modes, etc.

The temperature of the CPU can be assessed in a number of possible ways.The first is via a temperature sensor on the CPU. This provides a directmethod of temperature measurement. Another possible way of measuring thetemperature is by determining if the coolant starts boiling. Bubbles inthe coolant combined to the lower density of the liquid body willincrease the coolant level and constitute an indication that thetemperature of the CPU has reached the boiling temperature of thecoolant.

Yet another method to derive the CPU temperature is to observe the powerconsumption of the CPU, as shown at FIG. 9, in particular at steps 800′and 802′. Since the power consumption is directly tied to thetemperature of the device, the amount of electrical power is a goodindicator of how hot the CPU is. Note that the power consumption isessentially a predictor of the CPU temperature as it takes some time forthe temperature of the CPU to rise when a certain power consumptionlevel has been reached. In that sense, the power consumption is anindicator of what the temperature will be a certain amount of time afterthe observation.

Yet another possibility is to correlate the data processing load of theCPU to its temperature, as shown at FIG. 8, in particular at steps 800and 802. This is somewhat similar to the electrical consumption in thesense that the larger the data processing load, the hotter the CPU gets.Accordingly by observing the data processing load, it is possible topredict when the CPU will get at a certain temperature point.

The data processing load is fairly simple to measure since the operatingsystem that manages the CPU can report this information.

When relying on power consumption or on data processing load to predictthe temperature a mathematical model is used which correlates all thevarious parameters together. The mathematical model essentially takes asan input the current temperature of the CPU and the observed data loador power consumption and computes a relation between time and thethermal energy to be removed from the CPU in order to keep the CPU wellwithin its operational boundaries.

The mathematical model also takes into consideration the thermal removalability of the cooling system 10 at various modes of operation.Therefore, the mathematical model can pro-actively adjust the mode ofoperation depending on the cooling requirements.

For certain applications, the mathematical model can be complex becauseit takes into consideration heat production phenomena and also heatdissipation phenomena. A practical way to build the model is throughtesting. This can be done by operating the CPU at various dataprocessing loads or power consumption levels and for each leveldetermining the amount of heat absorbed by the coolant to keep the CPUat a predetermined temperature. Transient characteristics such as thelag time between the heat rise and the switch to a certain powerconsumption or data load level can also be determined by this practicaltest.

The mathematical model can thus be used to match the predictingcondition, i.e. the power consumption of data load processing to aparticular operational mode of the cooling system 10 that provides thecooling requirements the model computes for the CPU.

The cooling system 10 has at least two different operational modes. Themodes distinguish from one another on the basis of cooling capacity. Afirst mode of operation is a static mode of operation. In this mode, theprimary coolant takes up heat from the CPU by conduction and that heatis transferred, also by conduction to the jacket 24. The primary coolantis not circulated outside the main cooling chamber 12. Typically, thismode of operation is suitable for situations where the CPU operates atlow power levels and generates little heat. The static mode of operationalso consumes little electrical energy since fewer components need to beoperated by comparison to other modes providing increased coolingcapacities.

The second mode of operation is a dynamic mode where the coolant fromthe main cooling chamber 12 is circulated to the throughout the coolantpath. The dynamic mode has a higher cooling capacity than the staticmode.

FIG. 9 illustrates the process steps performed by the software logic todetermine which mode of operation should be used.

Step 900 in the flowchart is a decision step that determines if thecooling requirements of the CPU can be met by the cooling capacity inthe static mode. To perform the assessment, the mathematical model canbe used to predict the cooling requirements of the CPU, in terms ofamount of heat that needs to be extracted by the primary coolant. Thesoftware will measure the electrical consumption of the CPU or receivedata indicating what the data processing load is. The mathematical modelcomputes the corresponding cooling requirements. If the static modesuffices to meet those requirements the processing logic proceeds tostep 902, which designates the various control actions necessary tomaintain the cooling system 10 in the static mode of operation.

The assessment performed at step 900 is constantly repeated. When theheat generation capacity of the CPU is stable, the mode of operation ofthe cooling system 10 does not change. However, if suddenly the CPUprocessing requirements change significantly, the mathematical modelpredictions may also change and trigger the switch to another mode ofoperation of the cooling system 10. For example, if the CPU dataprocessing load is rapidly raised, the mathematical model predicting theheat generation behavior computes that the static mode of operation isno longer sufficient and triggers the dynamic mode of operation. In thisinstance, the processing branches to step 904 where the control signalscause the cooling system 10 to operate in the dynamic mode.Specifically, the control signals trigger the various pumps and valvessuch that primary coolant is circulated from the main cooling tankthrough the entire primary coolant circulation path including the buffertank section 16.

In the process above, the ability of the mathematical model to predictcooling requirements allows the cooling system 10 to anticipate thecooling needs and switch modes even before the predicted heat generationlevel has been reached.

Note that in the above example, the cooling system 10 has been describedin terms of discrete modes of operation. A possible variant is tooperate the cooling system 10 in a continuously variable fashion toprogressively vary the cooling capacity to meet the coolingrequirements. For example, the cooling system 10 is designed toconstantly circulate the primary coolant from the main cooling tankthroughout the primary coolant path, including the buffer tank section16. A change of mode in this example is considered an operational changethat allows the cooling system 10 to absorb more heat. For example, suchoperational change can be an increase in the flow rate of primarycoolant through the coolant path, so effectively the cooling system 10operates in a different mode.

Changing the level of coolant in the main cooling chamber 12 can alsovary the operational mode of the cooling system 10. This is performed bygenerating control signals from the input/output interface 706 to theelectric drive 94 to displace the piston 96. When additional cooling isrequired, the logic module thus commands the drive 94 to turn thejackscrew 92 such that the piston 96 is downwardly displaced, expellingcoolant from the cylinder 90 into the main cooling chamber 12 and thusraising the level of primary coolant in the main cooling chamber 12.

Varying the boiling temperature in the main cooling chamber 12 is yetanother possible way of modifying the operational mode of the coolingsystem 10. Varying the pressure in the main cooling chamber 12 altersthe boiling temperature. The flowchart at FIG. 14 illustrates thisprocess. Assume that the primary coolant starts boiling but from anoperational standpoint it is desirable to increase the boiling point andthus delay the phase change process. At step 1300 the logic determinesif boiling is now occurring. In the affirmative, the processing moves tostep 1302 where the control system increases the pressure in the maincooling chamber 12 which has the effect of increasing the temperature atwhich the phase change will be occurring. A pressure increase in themain cooling chamber 12 assumes that the primary cooling path can bepressurized. Such pressurization can be accomplished by injection ofinert gas from a pressurized source. For instance, the control systemoutputs a control signal from the input/output interface 706 to a gasvalve to direct the valve to open and inject inert gas in the maincooling chamber 12. The valve closes when the desired pressure in themain cooling chamber is reached.

The control system logic is also provided with a series of modules thatcan sense alarm conditions and notify an operator or perform a certainnumber of actions automatically in order to prevent damage to the CPU.

The logic works by processing the outputs of various sensors to read inreal time the parameters of operation of the cooling system 10. Morespecifically, the logic will read temperature, pressure and flow rateinformation at various locations in the cooling system 10 and it willcompare them to predetermined settings to determine if the coolingsystem is operating within normal parameters or an abnormal condition isdeveloping.

In a specific example, the logic will read the outputs of level sensorsthat measure the level of primary coolant in the main cooling chamber12. The level of primary coolant in the main cooling chamber 12 is notexpected to vary much, except if there is a leak in the system. Somevariation is expected as a result of evaporation of the primary coolantor when the primary coolant starts to boil which will have the effect ofa slightly increasing its volume, hence its level. If the logicdetermines that the level has dropped below a certain threshold, it canautomatically replenish the main cooling chamber 12 by generating acontrol signal to the electric valve 86 to allow primary coolant in themake-up tank 70 to flow to the main cooling chamber 12.

Similarly, the logic also monitors the level of primary coolant in thebuffer tank 200. If the level drops below a certain point until acontrol signal is generated to an electric valve to allow primarycoolant from the make-up tank 300 to flow into the buffer tank 200 andthe raise the level of primary coolant to its normal level.

The make-up tank 300 can also be used in emergency conditions to injectadditional primary coolant such as to increase the cooling capacity.This action may be performed even if the level of primary coolant in thebuffer tank 200 is at its normal level. The fast transfer of a primarycoolant from the make-up tank 300 allows increasing the overall primarycoolant volume, hence providing additional cooling capacity in certaincircumstances. Example of such circumstances include situations wherethere may be a leak in the system and emergency cooling is required toprevent the CPU from overheating.

The emergency transfer of primary coolant from the make-up tank into thebuffer tank 200 can be triggered as a result of a low level of primarycoolant in the heat exchanger tank 200, excessive temperature of theprimary coolants or any other condition that indicates a risk ofoverheating.

In order to allow a quick transfer of primary coolant from the make-uptank 300 to the buffer tank 200 a pump may be used to transfer theliquid or the make-up tank 300 can be pressurized to cause the primarycoolant in the make-up tank 300 to be expelled when a valve is opened.

When certain emergency conditions arise such as for example a rapid riseof the temperature of the CPU the logic is programmed to automaticallytake certain actions at the CPU level in order to prevent thepossibility of damage. One example is to completely shut down the CPU.This is an extreme measure that is to be avoided as much as possible,since shutting down the CPU will interrupt all data processing activity.If the CPU is working on some critical functions the loss of the dataprocessing capacity may be a significant downside. Still, thatpossibility is preferred over the alternative where the CPU is damagedand needs to be replaced.

FIG. 11 illustrates the process in general. At step 1000, the logicdetermines if an alarm condition which requires a control action at theCPU level is present. If such control action is necessary, it isgenerated at step 1002 and output via the input/output interface 706. Asdiscussed below, the control action can be a command to shut down theCPU or to reduce the data processing load on the CPU.

The shutting down of the CPU is done by generating a control signal viathe input-output interface 706, which is directed as an input to theoperating system managing the CPU. In response to this input, theoperating system will immediately terminate all processing activity andallow the CPU to cool down before it is brought back online.

A refinement to the approach described above is to allow the logic ofthe control system to throttle the CPU and thus control the rate of heatgeneration in order to match it to the cooling capacity. This allowsproactively reducing the data processing load on the CPU and avoidingthe necessity to shut it down completely. In a specific example, thelogic monitors the state of the various sensors and if it determinesthat the temperature of the CPU is raising it will, as discussed earliertrigger certain number of actions to increase the cooling capacity.Should the temperature continue to rise beyond the point were thecooling capacity is at its maximum, which may occur if there is somefault in the cooling system, the logic determines that thedata-processing load on the CPU needs to be reduced in order to matchthe generated heat with the currently available cooling capacity.

The logic module will generate via the input-output interface 706 acontrol signal that is directed to the operating system managing the CPUto indicate to the operating system that the data-processing load on theCPU is to be reduced. The degree of reduction of the data processingload is determined on the basis of the available cooling capacity. Thelogic module of the control system can compute the currently availablecooling capacity and determine on that basis how much data processingload is allowable without exceeding that capacity. A simple method ofimplementation is to provide the logic with a lookup table that matchesdifferent cooling capacities with the different data processing loads.It is thus a simple matter for the logic to map the currently availablecooling capacity with the corresponding data processing load and togenerate the control signal to the operating system to bring the CPUdown to the desired data processing load. The control signal couldconvey the desired data processing load such that the operating systemcan enforce it. Optionally, the computation of the allowable dataprocessing load for the CPU can be performed at the operating systemlevel. The control signal therefore would only convey the currentlyavailable cooling capacity and lets the operating system determine whatthe processing load should be in order to avoid exceeding that coolingcapacity.

In addition to controlling the operation of the cooling system, thelogic module also monitors the condition of the primary coolant todetermine if it is still fit for continued operation. In the event itscondition is not satisfactory, the logic module can notify the operatorsuch that the coolant can be replaced.

One parameter of the primary coolant that is being monitored is itsdielectric constant. Since the CPU and the associated electronicequipment is in direct contact with the primary cooling liquid, thecoolant has to have a sufficient dielectric capability to preventshort-circuits. The dielectric capability of the liquid medium canchange over time for various reasons. For instance, the primary coolantcan undergo a progressive chemical change that has the effect ofreducing its dielectric constant. Another reason is a presence ofimpurities. The Novec family of engineered liquids can act as solvents,thus solving impurities that may be present in the primary coolant path.When the impurities enter in suspension and are distributed throughoutthe liquid mass they may increase the conductivity of the liquid.Accordingly, over time, the conductivity of the primary coolant canincrease sufficiently to a point where it may allow some degree ofelectrical conduction between conductive terminals of the electronicmodule 18 that is immersed in the coolant. To avoid this problem, theprimary coolant is monitored for sufficient insulation capacity and inthe event it is found to be somewhat conductive, it is replaced.

The logic module will monitor the dielectric constant of the primarycoolant via a sensor. Different types of sensors can be used withoutdeparting from the spirit of the invention. One example is to use asensor which directly measures the dielectric constant of the primarycoolant. Such sensors operate on different principles, one being acapacitor type arrangement where an electric field is establishedbetween two plates which are spaced apart, the area between the platesbeing filled with coolant. Another example is to directly measure theconductivity of the liquid (electrical resistance measurement).

Advantageously, the insulation capacity of the primary coolant ismonitored constantly. This allows determining in real time if theprimary coolant can continue to provide the desired level of dielectricprotection. Should a problem be identified, the logic module can triggeran alarm to indicate that a replacement of the primary coolant isrequired.

For that purpose, the logic module contains predetermined operationallimits for the conductivity or dielectric constant of the primarycoolant which cannot be exceeded. These limits are thresholds and as themeasured conductivity or dielectric constant gets closer to them theseverity of the alarm condition is increased. This allows the operatorto plan ahead in performing the desired maintenance to avoid unplannedshutdowns.

Another possible refinement is for the logic module to determine trendsin the increase of the conductivity or reduction in the dielectricconstant of the primary coolant such as to be able to predict when aprimary coolant replacement will need to be done. The trenddetermination is done by collecting a series of conductivity ordielectric constant measurements over time. The rate of change of theseparameters allows determining when the thresholds will be reached. Forexample, assume that the cooling system is in operation for six monthsand the conductivity of the primary coolant has reached 50% of theallowable limit. The system can then predict that the threshold will bereached within the next six months.

The logic can thus notify the operator in a predictive manner that theuseful life of the primary coolant will be spent in a predictabletimeframe. In this fashion, the operator can see the amount of time leftbefore a primary coolant change needs to be performed.

The alarm itself can be directed to an operator console (not shown inthe drawings). The different alarm conditions can be designated indifferent ways on a GUI interface, the most urgent ones being shown inred while advisories are shown in a different color. In terms ofimplementation, the control signals generated by the input-outputinterface 706 are directed to a display device, which in this exampleconstitutes the operator console, to be displayed. In the case of theprimary coolant life counter, information could be displayed in adisplay field of the GUI in terms of amount of time left, percentage oflife spent or in any other way.

The flowchart at FIG. 12 illustrates the above process. At step 2000 thedielectric constant or conductivity of the primary coolant is measured.The results of the measurement are processed at step 2002 to determinethe type of action required. The processing step 2002 designates thecomparison of the measurements with thresholds to determine if an alarmcondition is to be generated and in the affirmative the severity of thealarm condition. The output action shown in the flowchart can alsoinclude an indication on an operator console of the computed remaininglife of the primary coolant and prediction as to when it will need to bechanged.

The logic module also analyzes the primary coolant for proper coolingcapacity. Over time, as the chemical composition of the primary coolantmay slightly change, the change can affect the ability of the coolant toabsorb thermal energy. The phenomena can, in turn alter the coolingcapacity of the system. For that reason, the logic module measures thecooling capacity and should that cooling capacity fall below athreshold, an alarm is raised to notify the operator that the primarycoolant needs to be replaced. The cooling capacity can be determined inindirectly. This can be done by placing in the fluid path of the primarycoolant a heat source such as an electric filament in a spaced apartrelationship with a thermal sensor.

The response of the thermal sensor to a predetermined thermal excitationproduced by the heat source when the primary coolant is fresh is known.The response essentially represents the ability of the liquid medium tochannel heat. As the primary coolant ages, that response will change andthe change indicates a loss of cooling capacity. The same approachdescribed above in connection with the conductivity or the electricconstant of the primary coolant can be used to generate alarm conditionsindicating to the operator that the liquid is undergoing coolingcapacity loss and may need to be replaced. The logic module compares thedetermined cooling capacity to thresholds and if they are exceeded, analarm is raised, trends can also be computed based on variations incooling capacity to predict when the primary coolant needs to bechanged. Control signals issued by the control system are displayed onthe operator console to indicate the amount of time left before coolantreplacement is required.

The flowchart in FIG. 13 generally illustrates this process. At step3000 the cooling capacity of the primary coolant is determined. At step3002, the results are compared to the cooling capacity of fresh coolant.Step 2004 determines the proper action to take depending on thedeviation. The action can be an alarm condition or computing anapproximation of the remaining life of the primary coolant.

1. A system for cooling a CPU, comprising: a. a tank for holdingdielectric coolant in a liquid phase and for receiving the CPU that isimmersed in the coolant; b. a cover for closing the tank; c. an electricpathway through the cover for communicating data signals to the CPU. 2.A system as defined in claim 1, wherein the electric pathway conveysdata signals and electric power signals.
 3. A system as defined in claim2, wherein the tank is a first tank, the system including a second tankfor holding dielectric coolant in fluid communication with the firsttank.
 4. A system as defined in claim 3, including a control device foradjusting a level of dielectric coolant in the first tank.
 5. A systemas defined in claim 4, wherein the control device for adjusting a levelof dielectric coolant is responsive to a sudden loss of dielectriccoolant from the first tank.
 6. A system as defined in claim 5, whereinthe control device for adjusting a level of dielectric coolant isresponsive to an increase in cooling demand for the CPU to introduce anadditional volume of dielectric coolant in the first tank.
 7. A systemas defined in claim 4, including a device for creating coolant flow inthe first tank.
 8. A system as defined in claim 7, wherein the devicefor creating coolant flow in the first tank includes a pump.
 9. A systemas defined in claim 7, wherein the device for creating coolant flow inthe first tank operates in response to an increase in cooling demand forthe CPU.
 10. A system as defined in claim 9, wherein the device forcreating coolant flow has a first and a second mode of operation, in thesecond mode of operation the device for creating coolant flow flowingmore coolant than in the first mode of operation.
 11. A system asdefined in claim 10, wherein the first mode of operation is a staticmode of operation wherein the device for creating coolant flow is notoperating.
 12. A system as defined in claim 1, including a device forfacilitating release of bubbles of dielectric coolant from a surface ofthe CPU.
 13. A system as defined in claim 12, wherein the device forfacilitating release of bubbles provides a Venturi effect.
 14. A systemas defined in claim 12, wherein the device for facilitating release ofbubbles includes a sound wave generator.
 15. A system as defined inclaim 12, wherein the device for facilitating release of bubbles impartsmovement to the CPU.
 16. A system as defined in claim 15, wherein themovement is an oscillatory movement.
 17. A system as defined in claim12, wherein the device for facilitating release of bubbles creates jetsof dielectric coolant against the CPU.
 18. A system as defined in claim1, wherein the dielectric coolant is a first coolant, the systemincluding a cooling path for circulating a second coolant, the coolingpath being in thermal transfer relationship with the tank.
 19. A systemas defined in claim 18, wherein the cooling path is in thermal transferrelationship with the cover.
 20. A system as defined in claim 1, whereinthe dielectric coolant has a first fraction and a second fraction, thefirst fraction having a different boiling point from the secondfraction. 21-57. (canceled)